A circuit board (sometimes referred to as a printed circuit board or a printed wiring board) is the basic building block for interconnecting electronic devices in a system. Electronic devices, usually integrated circuit (IC) devices, are mounted onto the circuit boards using a number of mounting mechanisms, such as by use of connectors or by directly mounting the devices onto a surface of the circuit board. A circuit board also includes the wiring required to interconnect the devices electrically.
The number and density of signal lines in a circuit board are continuously increasing due to the increased density of circuits that can be formed on each IC chip. The number of input/output (I/O) pins that exist on each IC chip can be quite large, which means that a large number of signal wires are needed to carry signals from one IC chip to another component in the system. To increase the density of signal wires that can be provided in the circuit board, a circuit board is usually formed of multiple layers. Some layers contain signal wires for transmitting signals, while other layers contain power reference planes, which are connected to ground or to a power supply voltage, e.g., a three-volt voltage, a five-volt voltage, or some other power supply voltage. In other arrangements of circuit boards, power reference planes are not used. To connect signal wires in different layers of the circuit board, vias are provided. A via is an electrical connection that is run through multiple layers of the circuit board to complete a signal path using different layers, or to provide an electrical connection to ground or power. Typically, the via is run generally perpendicularly to a main surface of the circuit board.
With large numbers of IC chips and signal wires (I/O circuits) in a circuit board, switching noise can be a problem during system operation, especially at high frequencies. To mitigate switching noise, surface mount technology (SMT) decoupling capacitors are commonly used. These capacitors are mounted to either the primary or secondary (top or bottom) surface of the circuit board, and connected to reference planes through vias. At high frequencies, a capacitor provides a low impedance bypass path for switching noise between the power supply voltage plane and the ground plane.
One issue associated with connecting decoupling capacitors to reference planes is the relatively high inductance resulting from the combination of the capacitor's package, a via, and the interconnecting structure from the decoupling capacitor to the via. As frequencies increase into the hundreds of megahertz (MHz) or gigahertz (GHz) range, the impedance associated with the combined inductance of each decoupling capacitor circuit becomes much larger than the capacitive impedance associated with the decoupling capacitor itself. To reduce the package inductance, surface mount technology (SMT) capacitors are used. To reduce the interconnection inductance, low-inductance interconnections are used, such as short wires, wide interconnects, multiple vias, and so forth. Nevertheless, because of the increased impedance caused by the inductance of the via, the SMT decoupling capacitor is unable to effectively provide a low-impedance bypass path for switching noise at high frequencies. In other words, because of a significant impedance introduced by the via inductance into the decoupling path, a capacitor loses its decoupling effectiveness in providing a bypass path for high frequency noise.
Other techniques have also been employed to provide decoupling capacitance in circuit boards. For example, an embedded capacitance in a circuit board has been employed to avoid effects of via inductances. However, conventional embedded capacitance techniques are typically associated with relatively low capacitance, which means increased impedance at high frequencies. Without effective decoupling, switching noise on a circuit board can cause device operation to fail under certain conditions.